# Conditions

Health conditions may be the most common type of health statistic. There are two ways to measure health conditions. Two ways to measure health conditions are the prevalence and the incidence of the disease.

Incidence refers to the number of new cases of disease in a given population. An incidence rate expresses the number of new cases of a disease divided by the total number of persons at risk for the disease.[1]  According to the World Health Organization (WHO), the number of people newly infected with HIV in 2015 was 29.2 cases per 100,000 population.[2] This is the global incidence of HIV.

Prevalence is the proportion of a population with a specific disease or condition. It is a measure of the total number of cases in a defined population at a particular time or during a given time period.[3] The World Health Organization (WHO) reported the global prevalence of HIV in 2015 at 509.7 per 100,000 population. This means that the total of new and existing cases of HIV in the world in 2015 was 509.7 for every 100,000 people (see the image below).[4] Because prevalence reflects the total number of existing cases at a point in time, prevalence may actually increase even if incidence rates decrease for a given illness. This could occur if people live longer with a disease.

Incidence and prevalence estimates may also be presented as counts of the number of new or total cases of an illness, respectively, in the population at risk. For instance, two additional ways of expressing the global prevalence of HIV in 2015 are by citing the number of people living with HIV in the world in 2015- this was 36.7 million- or as a percent of the 2015 global population: 0.51%.

When using health statistics, it is important to consider how to express information for a particular audience and purpose (percentage, rate, count). Percentages and rates are useful because they put numbers into context and provide standard measures that can be compared across populations. Together, incidence and prevalence represent measures of morbidity, or the disease state (illness) of a population.

The image below shows the incidence of different diseases, from 1960 to 2010.

[1] “Basic Statistics: About Incidence, Prevalence, Morbidity, and Mortality - Statistics Teaching Tools.” New York State Department of Health, n.d. Web 1/24/2017
[2] “HIV/AIDS Data and Statistics” World Health Organization (WHO), 2017. Web 1/24/2017
[3] “Basic Statistics: About Incidence, Prevalence, Morbidity, and Mortality - Statistics Teaching Tools.” New York State Department of Health, n.d. Web 1/24/2017
[4] “HIV/AIDS Data and Statistics” World Health Organization (WHO), 2017. Web 1/24/2017
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